Reinforced ceramic matrix composites are well suited for structural applications because of their potential toughness, thermal resistance, high temperature strength and chemical stability. These composites can be produced by the addition of whiskers, fibers or platelets to a ceramic matrix. The non-brittle nature of these composites provides the much needed reliability that is otherwise lacking in monolithic ceramics.
Fabrication of ceramic matrix composites reinforced with sintered continuous fibers is more difficult than fabrication of dense monolithic ceramics. Conventional sintering of a green ceramic matrix reinforced with sintered fibers is not possible if the green ceramic matrix has rigid inclusions. Densification can, however, be achieved by chemical vapor infiltration (CVI) or reaction bonding. Reaction bonding is the preferred method because it is less time consuming and more often produces a fully dense body than the CVI process. For high temperature applications, full densification is necessary to prevent rapid oxidation degradation of the reinforcements or reinforcement coating.
Densification by reaction bonding, described in U.S. Pat. No. 3,205,043 to Taylor, involves infiltrating molten silicon through the pores of a green body containing SiC and carbon. The silicon reacts with the carbon to form SiC, which then bonds the SiC grains together. In the absence of carbon, the infiltrated molten silicon solidifies upon cooling, thereby filling the pores of the SiC bonded SiC body. This process is known as siliconization. The resulting fully dense end product contains SiC and residual free silicon. Since reaction bonding does not involve shrinkage of the green body as does conventional sintering, the final dense product is a near net shape.
Fracture resistance of ceramic matrix composites is achieved through crack deflection, load transfer, and fiber pull-out. Fiber pull-out, which is well established as central to the toughness of ceramic fiber composites, is achieved by having little or no chemical bonding between the fibers and matrix. The fibers must be able to readily debond and slide along the matrix for increased fracture toughness of the composite.
It is known that many fiber-matrix combinations undergo extensive chemical reaction or interdiffusion between the fiber and matrix materials, each of which is likely chosen for the contribution of specific mechanical and/or physical properties to the resulting composite. Such reaction or interdiffusion can lead to serious degradation in strength, toughness, temperature stability and oxidation resistance. The fiber-matrix interface is therefore very important to preventing or minimizing chemical reactions and interdiffusion.
Surface modification of the fibers is an effective means to control the fiber-matrix interface. This can be accomplished by coating the fibers with a suitable ceramic to inhibit the fibers from reacting or bonding with the matrix. The ceramic coating allows the fiber to pull out from the matrix and slide along the matrix, thus increasing the fracture toughness of the composite.
Coated silicon carbide fibers and whiskers are known reinforcements for composite materials. U.S. Pat. No. 4,929,472 to Sugihara et al. discloses SiC whiskers having a surface coated with a thin, 7-100 .ANG., carbonaceous layer and SiC whiskers coated with a Si.sub.3 N.sub.4 layer which is 15-200 .ANG. thick. These surface coated whiskers are used as a reinforcing material for ceramics such as SiC, TiC, Si.sub.3 N.sub.4, or Al.sub.2 O.sub.3.
U.S. Pat. No. 4,781,993 to Bhatt discloses a SiC fiber reinforced reaction bonded Si.sub.3 N.sub.4 matrix wherein the SiC fibers are coated with an amorphous carbon layer and an overlayer having a high silicon/carbon ratio covering the amorphous layer.
U.S. Pat. No. 4,642,271 to Rice discloses BN coated ceramic fibers embedded in a ceramic matrix. The fibers may be composed of SiC, Al.sub.2 O.sub.3 or graphite, while the matrix may be composed of SiO.sub.2, SiC, ZrO.sub.2, ZrO.sub.2 --TiO.sub.2, cordierite, mullite, or coated carbon matrices.
U.S. Pat. No. 4,944,904 to Singh et al. discloses a composite containing boron nitride coated fibrous material. Carbon or SiC fibers are coated with BN and a silicon-wettable material and then admixed with an infiltration-promoting material. This mixture is then formed into a preform which is then infiltrated with a molten solution of boron and silicon to produce the composite.
Teusel et al. in "Aluminum Nitride Coatings on Silicon Carbide Fibres, Prepared by Pyrolysis of a Polymeric Precursor", J. Mat. Sci., 25 (1990) 3531-3534, discloses a method of coating SiC fibers. Nicalon (SiC) fibers, Nippon Carbon Co. Ltd., were thermally pretreated in nitrogen and dip coated with a solution of metallic aluminum in an organic electrolyte. The fibers were then calcined at 900.degree. C. under anhydrous ammonia. The authors found that a thin coating, about 0.5 microns, produced a smoother and more uniform surface than thicker coatings. The performance of these AlN coated SiC fibers in a SiC matrix was not discussed in the Teusel et al. article.
A specific problem encountered with SiC reinforced SiC composites is that the SiC fibers or coatings on the SiC fibers may react with the matrix during formation of the composite, resulting in a strong fiber-matrix bond. This strong interfacial bond leads to decreased fracture toughness. It is an object of the invention, therefore, to provide a process for incorporating SiC fibers into a SiC matrix while controlling the fiber-matrix interface to achieve high fracture toughness. It is also an object of the invention to provide a process for producing a SiC composite possessing high temperature strength.